1. Field of the Invention
The present invention relates to a lithium ion secondary battery, and, more particularly, to a lithium ion secondary battery having a shape memory safety vent adapted to discharge internal compressed gas.
2. Description of Related Art
A typical lithium ion secondary battery includes: an electrode assembly formed by winding a positive electrode plate having a positive electrode active material attached thereto, a negative electrode plate having a negative electrode active material attached thereto, and a separator positioned between the positive and negative electrode plates to avoid a short circuit and allow only lithium ions to move, into a commonly known “jelly roll” structure; an electrolyte for enabling lithium ions to move; a can for containing and sealing the electrode assembly and the electrolyte; and a cap assembly for covering the can and preventing the electrode assembly from escaping.
Such a lithium ion secondary battery is manufactured as follows. A positive electrode plate having a positive electrode active material attached thereto, a negative electrode plate having a negative electrode active material attached thereto, and a separator are laminated and wound into a jelly roll. They are placed into a square-type can and a cap assembly is welded to the top thereof to seal it; an electrolyte is injected. Charging and inspection are performed to complete a bare cell. Various protective devices are attached to the bare cell. Assembly and inspection are then preformed to complete a conventional battery pack.
The lithium ion secondary battery is charged in a static voltage/static current condition and no overcharging occurs as long as the charging voltage is correctly controlled by the charger. When the charger is damaged or erroneously operated, however, abnormal charging occurs and the voltage and temperature of the battery abruptly increase. Such an increase decomposes the positive electrode active material or the electrolyte inside the battery. As a result, gas is generated and the battery swells. Such gas generation and swelling phenomenon may also result from heat supplied from outside the battery. The generated gas increases the internal pressure of the battery and causes the electrolyte to leak out. The battery may then explode or catch fire.
Safety measures to prevent such problems include a positive temperature coefficient (PTC) thermistor and a separator incorporating a shutdown function, as well as a safety vent actuated by gas generation as mentioned above.
Particularly, the safety vent of a conventional square-type lithium ion secondary battery refers to a thinner region formed on the bottom surface of the can or on the cap assembly. The safety vent fractures when the internal pressure of the can reaches a reference level and discharges gas to the exterior. Once actuated, the safety vent cannot return to the original state (i.e., it is irreversible) and must be disposed of.
As mentioned above, conventional safety vents are actuated only when the pressure reaches a reference level, regardless of the temperature of the battery, in an irreversible manner. During overcharging, however, voltage rise is generally preceded by temperature rise (or the battery temperature rises due to heat supplied from the exterior), which is then followed by gas generation. Although actuation in response to temperature is ideal, conventional safety vents respond only to pressure. This is an obstacle to improving the safety of the battery.
A large deviation exists in the battery pressure which fractures a safety vent formed as a thinner region. In other words, a safety vent may be unnecessarily actuated below a reference temperature or pressure or may fail to be actuated even above a reference temperature or pressure. This may result in explosion or fire since the safety vent is not designed to be actuated in proportion to temperature, but is designed to be physically actuated within a predetermined pressure range.